The present invention relates generally to a variable-capacitance device and a voltage-controlled oscillator using the same.
A conventional variable-capacitance device of MOS type includes an N-well region; P+ or N+ type of source and drain regions, an oxide layer and a gate electrode. The P+ source and drain regions are formed in the N-well region. When a control voltage, which is higher than a threshold value of the device, is applied to the gate electrode, a channel is formed and a capacitor area is increased. The capacity of the device can be controlled by adjusting a voltage applied to the gate electrode.
When a channel area is changed in size, the gradient (rate of change) of variable capacity is changed. The gradient of variable capacity is changed when channel area is changed. When a channel area is small, the gradient of variable capacity is smaller or gentle. On the other hand, when a channel area is large, the gradient of variable capacity is larger or steep.
As described above, a variable range is changed when a gradient (rate of change) of variable capacity is changed. In other words, when a gradient of variable capacity is decreased, a variable range is decreased as well. On the other hand, when a gradient of variable capacity is increased, a variable range is increased as well. Namely, it is difficult to control a gradient of variable capacity without changing a variable range of capacity.
Accordingly, it is an object of the present invention to provide a variable-capacitance device, in which the gradient (rate of change) of variable capacity can be controlled without change of variable range.
Additional objects, advantages and novel features of the present invention will be set forth in part in the description that follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
According to a first aspect of the present invention, a variable-capacitance device includes first and second variable-capacitance elements which are connected in parallel to each other. Each of the first and second variable-capacitance elements include gate, source and drain regions and operates in response to a control voltage applied to the gate region. The first and second variable-capacitance elements have different threshold values.
The first and second variable-capacitance elements may be formed by fabrication steps comprising implanting impurities into a semiconductor substrate. The threshold values of the first and second variable-capacitance elements can be adjusted by controlling an amount of impurities to be implanted into the semiconductor substrate. According to another aspect of the present invention, the threshold values of the first and second variable-capacitance elements are adjusted by controlling lengths of the gate regions (electrodes).